Time domain reflectometers, TDR, provide a means to determine some characteristics of faulty and normal electrical transmission lines, by sending an excitation signal and receiving a reflected response with subsequent analysis.
FIG. 1 (Prior Art) shows, in general terms, the external appearance of a portable realisation of a TDR. Other realisations are possible, for instance a fixed installation wherein the TDR may be automatically controlled and would not require any user keypad or visual display.
FIG. 2 (Prior Art) shows the typical blocks that make up a TDR. These are:
Power Supply—which provides the necessary power to the various circuits
Processor/Memory—which as in many examples of modern instrumentation, provides overall operational control, processing of user actions, control of information provided to the user, management for the generation of test signals, management for the acquisition of measured signals, mathematical analysis of measurements and the application of signal processing algorithms. In this context, the term “user” might also mean a separate piece of linked system control equipment as well as a human operator.
User Interface(s)—in a portable TDR these would typically be a keypad for entering commands/data and a screen for the display of measured signals responses, derived measurements and system information. In fixed installation, the user interface might consist of serial communication port such as RS232 or USB.
Test Signal Generator—provides a test signal for application to the transmission line(cable) under test.
In practice it might be in the form of a voltage source or a current source. It might also be presented in either a single-ended (unbalanced) or a balanced form.
Traditional TDRs use a substantially rectangular pulse, or pulses that are smoother in nature such as a half-sine shape or a raised cosine shape. Other TDRs use a step waveform, which does not return to zero over the duration of the measurement. More complex waveforms may be used.
Line feed resistor(s)—these provide the correct matching impedance for the line being tested. When a signal travelling along a transmission line encounters a change in characteristic, a reflection occurs. This is also true for a reflected wave returning to the TDR instrument. The instrument should therefore present an impedance characteristic sensibly close to the impedance characteristic of the line under test, if it is to avoid causing further unwanted signal reflections.
The line feed resistor(s) are therefore provided to give the correct matching characteristic for the Line under test. Multiple selections may be provided to cater for various Line types.
In practical realisations, the Test Signal Generator might consist of a voltage generator which is then arranged in series with the Line Feed Resistor(s) or a current generator arranged in parallel with the Line Feed Resistor(s). These are equivalent as per the well-known Thevenin and Norton equivalent forms.
Also the signals might be provided (and responses measured) as either a single-ended or a balanced form which are well known in measurement systems. The later analysis is presented in the single-ended (unbalanced) form although this is easily extended to the balanced form, as is well known.
Additionally, some form of dc isolation might be provided between the TDR instrument circuitry and the connectors providing the access to the Line (cable) Under Test. Typically this is done by the use of capacitors whose value is chosen to have minimal effect on the signals generated to and received from the Line (cable) Under Test. If these capacitors do have a significant effect, it can be compensated for by use of traditional analogue or digital filter techniques.
Signal measurements—this block provides the ability to capture the electrical signals appearing on the TDR Line (cable) Under Test, access point. It can therefore acquire signals with or without the cable actually connected. Typically an input amplifier of suitable impedance when considered in conjunction with the Line Feed Resistor(s) will pass the signal to an Analogue to Digital Converter (ADC) which is used to capture signal values on a point by point basis in time, which are then passed via the processor to a memory store for later evaluation.
In a practical TDR, the effect on the measured signal due to any dc isolation may again be compensated for by use of traditional analogue or digital filter techniques. Also, the input amplifier/ADC circuit may be presented in either single-ended (unbalanced) or balanced configurations.
Access point—provides the terminal connections such that the cable under test can be connected to the instrument's test and measurement circuitry.
Whenever a signal travelling along a transmission line encounters some change (discontinuity) in the line characteristic, a portion of the signal is reflected back towards the sending end of the line. The nature of the reflection signal is determined by the discontinuity characteristic, which might be anywhere between a short circuit and an open circuit.
The primary objective for a TDR is usually to estimate the physical location of a line discontinuity such that it can be repaired or replaced if the unwanted effect is severe enough. A second objective is thus to determine the nature of the discontinuity and hence provide an estimate of the degree of severity for any unwanted effects that it may produce.
In broad terms, it is accepted that a reflection of the same polarity as the applied test signal denotes a fault impedance higher than the natural impedance of the transmission line. Conversely, a reflection of opposite polarity to the test signal indicates a fault impedance that is lower than the natural impedance of the transmission line.
The amplitude of the reflection provides an indication as to the severity of the impedance mismatch caused by the fault, with small mismatches producing small reflection amplitudes and large mismatches such as short or open circuit conditions producing larger reflections. Whilst the relative size of the reflection amplitude is dependant on the fault characteristic, the absolute value is also dependant on how far away the fault is and how much signal loss is introduced by the cable on the signal as it travels to and from the fault location.
Prior art techniques include the use of simple cable loss models together with measurements of test signal and reflection signal timings and amplitude measurements to provide estimates of the fault location and the fault characteristic. Such methods will always be limited in that they only provide a rough approximation to the actual physics of the signals in the transmission line.
The limitation of these simple techniques is due to the fact that they do not accurately take into account the detailed frequency-dependant transmission loss, transmission delay and cable impedance characteristic that exists in real cables.
FIG. 3 illustrates some simulated typical responses for a variety of fault conditions.
FIG. 3a shows the Line signal measured at the TDR, consisting of a test pulse and reflections for an open circuit at two different locations. As expected the high impedance fault produces a reflection signal of the same polarity as the test pulse. Also as expected, when the fault is further away, the reflection is smaller in amplitude due to additional loss due to the greater length of cable.
FIG. 3b shows the Line signal measured at the TDR, consisting of a test pulse and reflections for a short circuit at two different locations. As expected the low impedance fault produces a reflection signal of opposite polarity to the test pulse.
FIG. 3c shows the Line signal measured at the TDR, consisting of a test pulse and reflections for a 200 ohm fault at two different locations. Given that in this example the nominal cable impedance is 100 ohms, then again we see a reflection signal of the same polarity as the test pulse indicating a higher impedance fault.
FIG. 3d shows the Line signal measured at the TDR, consisting of a test pulse and reflections for 110 ohm fault at two different locations. Again, the nominal cable impedance is 100 ohms.
On this occasion, the simple rules for high and low impedance faults are no longer sufficient to evaluate the location and the nature of the fault.
In the first instance, the reflections are substantially opposite to the test pulse, indicating a low impedance fault. Secondly, the substantial portions of the reflections start significantly later in time than say the reflections for the 200 ohm faults seen in FIG. 3c, even though the faults occurs at the same two locations. In a practical situation where measurement noise could obscure the fine detail of the FIG. 3d traces, the fault would be falsely determined as a low impedance fault and at a positions later in time (further in distance) than are actually the case.
The failure to use an adequate description of the processes, can lead in this case to both the nature and position of the fault to be miscalculated.
More recently, frequency-domain analysis using FFTs has been applied to certain TDR measurements, to provide enhanced analysis.